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diff --git a/third_party/libwebrtc/rtc_base/strings/string_builder.h b/third_party/libwebrtc/rtc_base/strings/string_builder.h
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+++ b/third_party/libwebrtc/rtc_base/strings/string_builder.h
@@ -0,0 +1,170 @@
+/*
+ *  Copyright 2018 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef RTC_BASE_STRINGS_STRING_BUILDER_H_
+#define RTC_BASE_STRINGS_STRING_BUILDER_H_
+
+#include <cstdio>
+#include <string>
+#include <utility>
+
+#include "absl/strings/string_view.h"
+#include "api/array_view.h"
+#include "rtc_base/string_encode.h"
+
+namespace rtc {
+
+// This is a minimalistic string builder class meant to cover the most cases of
+// when you might otherwise be tempted to use a stringstream (discouraged for
+// anything except logging). It uses a fixed-size buffer provided by the caller
+// and concatenates strings and numbers into it, allowing the results to be
+// read via `str()`.
+class SimpleStringBuilder {
+ public:
+  explicit SimpleStringBuilder(rtc::ArrayView<char> buffer);
+  SimpleStringBuilder(const SimpleStringBuilder&) = delete;
+  SimpleStringBuilder& operator=(const SimpleStringBuilder&) = delete;
+
+  SimpleStringBuilder& operator<<(char ch);
+  SimpleStringBuilder& operator<<(absl::string_view str);
+  SimpleStringBuilder& operator<<(int i);
+  SimpleStringBuilder& operator<<(unsigned i);
+  SimpleStringBuilder& operator<<(long i);                // NOLINT
+  SimpleStringBuilder& operator<<(long long i);           // NOLINT
+  SimpleStringBuilder& operator<<(unsigned long i);       // NOLINT
+  SimpleStringBuilder& operator<<(unsigned long long i);  // NOLINT
+  SimpleStringBuilder& operator<<(float f);
+  SimpleStringBuilder& operator<<(double f);
+  SimpleStringBuilder& operator<<(long double f);
+
+  // Returns a pointer to the built string. The name `str()` is borrowed for
+  // compatibility reasons as we replace usage of stringstream throughout the
+  // code base.
+  const char* str() const { return buffer_.data(); }
+
+  // Returns the length of the string. The name `size()` is picked for STL
+  // compatibility reasons.
+  size_t size() const { return size_; }
+
+// Allows appending a printf style formatted string.
+#if defined(__GNUC__)
+  __attribute__((__format__(__printf__, 2, 3)))
+#endif
+  SimpleStringBuilder&
+  AppendFormat(const char* fmt, ...);
+
+ private:
+  bool IsConsistent() const {
+    return size_ <= buffer_.size() - 1 && buffer_[size_] == '\0';
+  }
+
+  // An always-zero-terminated fixed-size buffer that we write to. The fixed
+  // size allows the buffer to be stack allocated, which helps performance.
+  // Having a fixed size is furthermore useful to avoid unnecessary resizing
+  // while building it.
+  const rtc::ArrayView<char> buffer_;
+
+  // Represents the number of characters written to the buffer.
+  // This does not include the terminating '\0'.
+  size_t size_ = 0;
+};
+
+// A string builder that supports dynamic resizing while building a string.
+// The class is based around an instance of std::string and allows moving
+// ownership out of the class once the string has been built.
+// Note that this class uses the heap for allocations, so SimpleStringBuilder
+// might be more efficient for some use cases.
+class StringBuilder {
+ public:
+  StringBuilder() {}
+  explicit StringBuilder(absl::string_view s) : str_(s) {}
+
+  // TODO(tommi): Support construction from StringBuilder?
+  StringBuilder(const StringBuilder&) = delete;
+  StringBuilder& operator=(const StringBuilder&) = delete;
+
+  StringBuilder& operator<<(const absl::string_view str) {
+    str_.append(str.data(), str.length());
+    return *this;
+  }
+
+  StringBuilder& operator<<(char c) = delete;
+
+  StringBuilder& operator<<(int i) {
+    str_ += rtc::ToString(i);
+    return *this;
+  }
+
+  StringBuilder& operator<<(unsigned i) {
+    str_ += rtc::ToString(i);
+    return *this;
+  }
+
+  StringBuilder& operator<<(long i) {  // NOLINT
+    str_ += rtc::ToString(i);
+    return *this;
+  }
+
+  StringBuilder& operator<<(long long i) {  // NOLINT
+    str_ += rtc::ToString(i);
+    return *this;
+  }
+
+  StringBuilder& operator<<(unsigned long i) {  // NOLINT
+    str_ += rtc::ToString(i);
+    return *this;
+  }
+
+  StringBuilder& operator<<(unsigned long long i) {  // NOLINT
+    str_ += rtc::ToString(i);
+    return *this;
+  }
+
+  StringBuilder& operator<<(float f) {
+    str_ += rtc::ToString(f);
+    return *this;
+  }
+
+  StringBuilder& operator<<(double f) {
+    str_ += rtc::ToString(f);
+    return *this;
+  }
+
+  StringBuilder& operator<<(long double f) {
+    str_ += rtc::ToString(f);
+    return *this;
+  }
+
+  const std::string& str() const { return str_; }
+
+  void Clear() { str_.clear(); }
+
+  size_t size() const { return str_.size(); }
+
+  std::string Release() {
+    std::string ret = std::move(str_);
+    str_.clear();
+    return ret;
+  }
+
+  // Allows appending a printf style formatted string.
+  StringBuilder& AppendFormat(const char* fmt, ...)
+#if defined(__GNUC__)
+      __attribute__((__format__(__printf__, 2, 3)))
+#endif
+      ;
+
+ private:
+  std::string str_;
+};
+
+}  // namespace rtc
+
+#endif  // RTC_BASE_STRINGS_STRING_BUILDER_H_